1. Field of the Invention
The present invention relates to identifying a periodic noise signal and removing the noise signal from a received signal stream when a desired signal is present. More particularly, it relates to a method, apparatus and article of manufacture for removing display interference from signals received from a magnetic head associated with a magnetic ink character reader.
2. Description of the Prior Art
Magnetic Ink characters are characters or symbols which are printed on a document in magnetic ink. Documents containing magnetic ink characters are typically bank checks, deposit slips and other documents. The magnetic ink characters may be printed in a standardized format or font (e.g., "E13B" or "CMC7" font) and are typically located near an edge of a document.
The magnetic ink characters are read by a magnetic head as the head moves across the document or the document is caused to move pass the head. The magnetic signals "picked up" by the head are converted to electrical signals which are processed by a magnetic ink character recognition unit which outputs the characters typically in ASCII or EBCDIC format.
Document processing systems with a MICR capability can be affected by magnetic interference which slows or prevents magnetic ink characters from being read. Magnetic coils such as for example electromagnetic coils generate magnetic fields resulting in interference relative to the magnetic information stored in the magnetic ink portion of a document. These interference signals may arise from nearby equipment such as displays, scanners, the MICR processing system itself, or motors and other electromechanical devices that may be used to move documents, etc.
This is particularly a problem in Point-of-Sale (POS) environments where MICR readers are typically located very near a check out display and where MICR information is obtained from checks tendered by customers. Due to lack of aisle space or the desire to more efficiently utilize floor space POS systems deployment usually result in the MICR or MSR device in close proximity to a CRT or other POS peripheral devices. This is particularly important when a MICR read head is located close to a display device because the display interference caused by the flyback or the retrace signal from the operation of the CRT. The coil from the CRT produces a significant source of noise to the MICR read head. Most POS systems also permit the display to be moved which changes the noise picked up by the magnetic read head.
One prior art solution to this problem is to use shielding. The source of the radiation, such as a display or coil, is shielded or the magnetic read head itself is shielded or partially shielded. Shielding can be used on the source or the pickup or a combination of both can be utilized. Because complete shielding is not possible, shielding does not eliminate the interference problem but, merely reduces the interference signal picked up by the magnetic read head. Furthermore, shielding is costly to implement given the high cost of the metal shielding material used and the extra steps required to apply shielding during the manufacturing process. Shielding can also cause other problems such as heating or overheating of components.
Another prior art solution is to add a second magnetic read head that is used to measure the magnetic interference signal. The electrical signal generated by the second magnetic read head can then be subtracted from the signal containing both the magnetic interference and the MICR characters. This solution requires a second read head and a means for subtracting one signal from the other. Additionally, this technique is imperfect because the two read heads cannot occupy the same physical space so that the interference signal pickup by the second read head may be substantially different then the interference picked up by the first magnetic read head. This is because the magnetic interference signals are different at the two locations. Thus, the use of two heads can result in self-interference.
These unresolved problems and deficiencies are clearly felt in the art and are solved by the invention in the manner described below.